We investigate the use of Synthetic Aperture SONAR (SAS) for the buried object imaging. In the sediment, the sound wave is strongly refracted because the sound speed is distributed by layer. The refraction causes the time shift Δt in the received acoustic signal, then the SAS image is distorted. First, we examined the effects of Δt on SAS image. We calculated sound wave in the sediment layer using Biot-Stoll model. The Δt calculated is applied to the ping signal of the receiver. After the synthetic aperture process, the resolution of the image is quantified for different types of sediment porosity β. Second, a compensation technique is proposed. The technique enhances the contrast of buried object in SAS image by giving error amount Δt as time delay in the SAS process. The results of the application of the technique reveals that the improved SNR (Signal to Noise Ratio) is more than 20dB.

View full abstract

The phase conjugate sound field with the deep sound source and the shallow array was simulated in underground. The phase conjugate beam aimed to a sound source of deep depth was formed also even by a short array in the vicinity of the surface. In addition, it is necessary to examine pulse shape formed in the vicinity of the sound source in the time domain.

View full abstract

Intense orange Na^* emission was observed in different spatial locations from blue emission during multibubble sonoluminescence in sulfuric acid. The color change from blue to orange observed along the streamer in the filamentous structure of a bubble cloud. By stroboscopic observation, the Na^* emission seemed to occur when a large bubble ejected tiny bubbles toward a pressure node after bubble coalescence around a pressure antinode. Comparing a high resolution Na^* spectrum of sulfuric acid with water, the full width at half maximum of the spectra were almost the same, where the estimations of the temperature and pressure inside the bubbles were 3300K and 390atm. The intensity of Na^* emission in sulfuric acid increased at lower frequency in contrast to the water case.

View full abstract

Ultrasonic atomization (USA) can be induced by applying ultrasound of high frequency, i.e., on the order of MHz, to a gas-liquid interface from the liquid underneath. It will preferentially occur, due partly to its high directivity, in comparison to acoustic cavitation induced favorably under lower-frequency (<MHz, thus technically-easier-to-attain larger-amplitude) conditions. When the liquid is subjected to high-frequency (&ge;1MHz) ultrasound, the atomization starts almost instantaneously in association with liquid jet formation and breakup. The atomization sequence, including the dynamics of interfacial oscillations, is visually analyzed via high-speed imaging. Selective ethanol separation from its aqueous solution is attempted through USA coupled with mist recovery by two-stage cooling. It is found that the USA process could be more closely triggered by sudden increase in the surface roughness of microscale, which would be viewed as localized surface patches of two-dimensional capillary waves, often associated with contraction-expansion sequence of the surface topology. Such surface patches can then lead themselves to further instability to generate a swarm of liquid droplets of microscale around the expanded phase of liquid column. Two separate recoveries, in the 1st and 2nd stages, with lower and higher ethanol contents, respectively, can be realized by optimizing-from an energy-saving perspective-cooling temperatures and carrier-gas flowrate; regarding the latter operating parameter, ethanol-rich mist consisting of small-size droplets tends to be carried selectively in favor of lower gas flowrate. The two-stage cooling process has thus an advantage of obtaining desired product by virtue of collectivity and transportability, respectively.

View full abstract

A palm-sized hydrogen sensor using ultrasonics is demonstrated at temperatures from -10 to 50℃. The sound velocity of hydrogen is about 1310m/s and the sound velocity of dry air is about 344m/s. Therefore, it is possible to measure hydrogen concentration by measuring the change in sound velocity when hydrogen is mixed with the air. However, the sound velocity also changes with temperature. In this study, we show that it is possible to detect hydrogen concentrations even when the temperature is varied by compensating the temperature effect.

View full abstract